Systems for supplying hot melt adhesive is rendered molten, a nozzle through which molten adhesive is supplied from the melt chamber by means of the pump, and heating means for maintaining the nozzle at an appropriate temperature for ensuring that the adhesive in the nozzle remains molten not only during an adhesive-applying operation but also between successive such operations, wherein, at the end of an adhesive-applying operation, the molten adhesive in the nozzle is caused to be sucked back thus to avoid drooling of adhesive from the nozzle.
One such system is described in our co-pending European Patent Application No. 83306752.3, for use in a so-called thermo-cementing and folding machine. It has been found, however, that despite the provision of a suck-back facility, if a next cycle of operation of the machine in question is not initiated within a reasonable time of the termination of the preceding cycle, then `drooling` of adhesive from the nozzle may nevertheless occur.
Examination of this problem has led to the consideration that whereas the initial or primary suck-back is effective to overcome the pressure creating the adhesive flow and to suck back into the nozzle the relatively large amount of adhesive which is collected about the nozzle outlet at the end of the adhesive-applying operation, nevertheless thermal expansion of the adhesive in the nozzle may take place when the adhesive remains for longer periods in contact with the nozzle, which is heated. Increasing the amount of adhesive initially sucked back in order to accommodate such expansion increases the risk, however, that air will also be drawn into the system with consequent detriment to the satisfactory operation of the system, which should provide a reliably uniform supply of adhesive to workpieces being coated.